Electron-emitting device and method of preparation



ELECTRON EMITTING DEVICE AND METHCD 0F REIARATION Fled'Aug. 11, 1921 l Vllllllll, .Y

Inventor a Irving Langmuir His Attotmy 0&5? IN 313/176 Patented Mar. 10, 1925,V

4imi'rjnn sm-frias i uznvmefnmvemurnfor sommer-Any., .NEW vomi, :Assmimn'fro eENEnlnLnLEc'rRIc i conmutar, mcononAfrIoN oF `NEW vonk.

.nLEc'rnoNenm-Irnim nnvrcn AND '-mzfrnon '.011' Pmmfrron -App1i'cat1on-ii1'eu--Augst 11, 11921. Asum1 No. 491.6834.

Totill whom t my 'ao/noem: V.` j.

"Be i lknown thatl, IR'vINo `vL'Aieis'njn'7:,1..a citizen of the United States, residingat Schenectady,.in the county of'Schenectady, .State of New York, .have invented, certain' new Aand useful Improvements in Electron? .Emitting Devices and lllethofds" off lepfarit:l tion, .of whichjhe .following-isa;.specica tion. l. This .invention .comprises .an improvement in .theaelectron emitting cathodes .of the type described in my prior S. Patents Nos. 1,2..11=,2l6\and.1,244,211.

aration `'and aclvfantf'igfes .of .a vacuum tube v containing-ac athode adapted to lbe operated at incandescence and containing distributed throughout its `inass thorium (or a com*v pound of thorium) as a-,constituent This type of electrode .consists of a refractory conducting material, .such for example .as tungsten, With-an addition of amaterial capable. of .increasing the electron emissivity Aof the cathode, such for example as'thoria, 'the latter being subjected to modifications during the preparation ofrthefcathode inthe electrondischarge device so that the active materialin a. finished device. accordingto all indications, consists of metallic .thoriunror at least of a. lower oxide of thorium .than thoria (Th02)- The cathode preparedaccording-to the methodsv set forth in my irst 4Patent'1,244216 which is 'commonly known as a. thoria'ted cathode,`is very sensitive to traces of oxidizing gas inthe bulb, as for example, .small traces of Water vapor which. may be given off from the Walls of the bulb during the normal operation of the device.

In order to prevent loss ofactivity of the cathode, I havedescribed-in my second xPatent No. 1,24E4,217, the beneficial results lobtained by "the admission .to the bulb .of la small amount of alkali .metal, such .as p0- tassium or sodium, or ofahydrocarbonzcompound,

is 'basedon the discovery thatcarbongin a. cathodeof .'the-tlioriated type, when present .in adefinitely limited amount, willprevent 0 .not only reoxidation of .active-.thorium .ma-

terial `but what is .more important, will act b'eneficiallyto reduce thoria to the .active state. 'Although .the .thoriated cathodes described .in my prior patents can be used Inthese ypatents I have describdftheprep-Q y 4ivith satisfactory results, lIhave founilthat4 'ag further improvement in- .regularity and Stability "can "be .effected :by using carbon fas hereinafter described, not vonly as in accordance with 'my prior patent in restrictedv quantities, as fan oxidation preventing agent,

but., as here'inat'er described, as an lngredi- 'lent' .of 1.a thori'atefd cathode.

.nfaccqrdance with.another .eature .only invention .my :improvedcathode 1s used 1n .conjunction vvith .0a Anoncarbonaceous reduc- Vingagent which .1s provlded 1n'tl1ebulb, .as

'thoriated .cathode and a positive electrode in .the presence of ionizableggas, .then .the benecial efectof thethorummay belargely ox-.entirely .destroyed by positive ion`.bom bardment. `Although .the alkali metals are entirely satisfactory as agents'to remove deleterious gases when the operating tem-- perature of the bulb is .only moderately above roomftemperature, as in the caseof radio receiving "tubes, '.the'se alkali .metals have a suliiciently )high vapor`jpressure' at the Aoperating temperature ol power devices 4in'vvhi'ch cathodes of the 'thoriated type are usedftowbecome ionized at .higher voltages and to cause Kremoval of the active lfilm iof thorium from the catho'deby'bombardment.

I "have` ifound that it is advantageous in manycases to provide a device kcontaining a'cathode ofthe thorated 'type with a 'materialfhaving an aiiinity for gases, such as magnesium, for example, which at the operating temperature of the device. has substantially no vapor pressure. My linvention will be pointed outwith'greaterpar- .-'ticular-ity :in Ithe accompanying claims tand ,described 'fully in conjunction with the. accompanying v lrawine:` which` shows .somel l p l .p Y)vliet'diagrammatically. .Anapparatusgsel My present invention in one.. of .its aspects for carrying out thepreparation A'offanincandescentcathode 1; rectiiier.

.a .bulb .l containingjza .flamentaljymethode; 2L

into rvwhichthoria has .been-.introduced as described in my L abovsementioned ;prior pat- \ent s,.and .an zano'de),iconsistingcof tungsten,

, I l .i

l so controlled by one ,or more constrictions, such molybenum or other `suitable material.

`These Velectrodes are,` respectively `connected to lead Wires 4, 4 and 5' sealedy into the Walls of they bulb. y"Ihe bulb is connected to the usual 'pumping system by;` means of a 'tubulature 6. The bulb also communicates withl an elongatedy tube 7 .adapted to be surrounded by a'y freezing-mixture contained" Within a double-Walled receptacle 8.y

In' carrying out my invention, t-he bulb 1 isv li'rst` baked out as usual to remove water vapor and is then exhausted. Within the tube 7 is placed :asubstance adapted to give a low pressure ofcarbonaceous vapor, as for example, anthrarcene` (GMHIO) or naphtha- When the vaporv pressure of the substance employed is relatively high, I prefer to rei duce the pressure ofthe khydrocarbon vapor in the-bulb l by surrounding the-tube with a freezing agent contained in the receptacle 8..

When employingy naphthalene, which has a moderately high vapor pressure, I prefer to reduce the temperature-of the containing tube 7 to about-20 to 30"- C. to reduce the'vapor pressure to about 0.1to 0.01 microns. The bulbl .which communicates with the side tube-7, therefore, ,is filled with hydrocarbon'vapor at ap roximat'ely this pressure. i The quantity o hydrocarbon may be Y as 9, l0 which may regulate the pressure with or Without a freezing agent.

at The filamentv 2, which preferably con- *sists of metallictungsten, containing about f 3/4%"thoria, isheate'd to a temperature of about2300o K. (2000o 0.)" for a few minl' utes, in contact'vvvith the hydrocarbon vapor.' AAt thistemperature th'ef incandescent filament -decomposes thev hydrocarbon vapor which comesin Contact 'with the incandesy'cent surface of thejtungsten and causes the chemical combination with the tungsten of @bide (WZC). The heating is continued for.

i in resistanceof Iabout 15 to 25%y of the* tungsten filament when cold, orat ro'onrv temperature.` This change inxresistance cor-l respondsto the presence o'fabo-uttwo-thirds the carbon set) free from the naphthalene vaporfwith the lformation of tungsten cara length of time suiicientto give an increase v per cent of carbon by Weightinfthe'filament.

The carb-on `content. most,y beneficial"'will vary withvtheconditions.v In vthe caseot a thoriated tungsten catho-de all ofthe metallic tungsten will beconverted. to brittle'.

- timgsten carbide (W2C)fivith`a carbon content 'as high asf'o. Ordinarily a carbon content ask high asabout 1% is 'as'high as'- permissible Without unduly weakeningthe 1 cathode.

' minute amounts of 'carbon vis highly undesi'r'y ablecin an incandescent lam a commercial- 'Although the presencel of even ly long life can be obtaine in a thoriated cathode containing carbon,4 as the'cathode ,discharge device.

, f i l need be operated only at a relatively lon' temperature, say at aboutthe operating temperature of an .ordinary carbon lamp, the electron emissivity of, a thoriated cathode being sol much greater' than that of an ordinary pure tungsten cathode. The hydrocarbon vapor is removed from contact with the filament either'b-y sealing off the tube v7 at the contraction'9 or cooling the sidetube 7 by mea-ns of liquid air in the receptacle 8 toa temperature so low that the vapor pressure ofthe naphthalene is negligible, thereby removing the naphthalene vapor from the bulb 1.- y

In some cases carbon to a desiredpropor-l tion may be combined Withthe cathode prior to lthe introduction of thev cathode into the bulb 1. For example, a thoriated filament may be prepared by squirting tungsten and thoria with a carbonaceous binder in amounts adapted to leave a suitable reenough carbon remains subsequently to fui-,

nish a supply ofactive thorium material by reduction'of the thoria. f

After the cathode has been provided with a contentof lreducing agent a non-carbonaceous .material capable of vcombining with watervapor and other deleterious gases may be introduced into the bulb. In some cases `this non-carbonaceous reducing agent may consist ofvalkali metal, such as sodium or potassium. Ciesium may be used toiad- /vantage in some cases, especially when some positive ionization isdesired in the electron siumjgcan be `conveniently provided by adding amixture ot' caesium halide and metallic calcium as a pasteitosomepart of-tll bulb, this method V: of introducing an' alkali metal being claimed -in an ,applicatiom SerialNo. (308,311, De-

cember-T214..1922,;by Ernest Charlton.

lVhen not desiring ionization of residual lgasesaor vapors,Ipreferlfdto introduce for' the .reasons given above. a lreagent having a Alowervapor pressure than the alkali metals,

as for example, an alkaline earth metal such as.Y magnesium, calcium, strontium, or' the lik'e. kConveniently'thisis done by prov1d` ing Within the bulb a small heating coil- 11. consistingof tungsten or other suitable refractory material.V and containing a' small quantity of the metal to be vaporized. Pref- 'erably 'il empioy metallic magnesium. By

magnesium, or'the vflike, in order to combine chemically with what little gas may have gotten into the bulb duringthe sealing otf operation. l t The'lmof magnesium, or-other equivalent metal, onv the -wal'ls off the bulb-Will coni- .K. to fiiolat lirev any 'thorium oxide onits bine "with anywate'r Vvapor or oxygen 'which 'maybe given olf 'by the bulb, or any-other parts ofthesdevice during'itsfoperation, and r thereby prevent an "oxidizati'on of the active thorium material upon the surface "of the cathode.

VThe vtungsten cathode 2 is prefer-ably heated 'for a 4'very' short time to about 2700J Asa final'step,thef cathode is activated,

'that is, brought into its highly active state by Ilie'ritingtoa temperature of about 2200o l. "for 'a vfew minutes. This results 'in the during the operation `of -the device.

reduction of thoria-bythe earbonva'nd causes the active thorium product to 'come -to the surfaceof thel'cathode Where it will beef- :fective in increasing the :electron emission The tube V is fthen rea-dy for operation.

As explained-in 'my prior patents, a temperature materially'higher'than the activatingtem'perature -Wll result in the 'thorium being vlatilized from fthe'suriace of -the cathode. A. fresh supply can. 'be produced and lcausedto accumulate uponft-hesurface by again activatingfor"fformingfan electrode at about 2200 K. Baring normalop- -erationythe cathodeis heated 5to a temperature below about 2200O lK.and as high-as- 2100o fK. Under these conditions, the elec-` tron emissionstays constant.

'What 'I claim as' new and desire vto :secure fect of gases/thana cathode 'body iunprovided `with carbon; l

.2. Inan electron discharge devicefa tung,-

-incandescence 'comprising' tungsten, about three-'quarters per cent jfthoria and about two- 4thi-rds perl-cent by Weight of carbon.

4. The method of increasing the'electron emiss'ivity .of an electrode of the thoriated "type which consists -inintr'oducin car-bon intos'aidelectrode in amounts insu cient to convert said metal to a'carbide, sealing said lectrode intofaspace substantially free from carbonaceous va ors'and thereupon 'activating' saidelectro e.

:5. An electric discharge device comprising an e'vacl'lacte'denvelope, electrodes therein4 one of whichv comprises a refractory metal, 'thorium oxide-and `lcarbon in an amount insuiiicjientto con-vert said metal to a carbide, Landia'jfnon-carloonaceous reagent capable fof jconlbining lviith deleterious gases in said iiig'anenvlpe, an anode and a cathode of :the thritd'type and a non-volatile material, which is capable of reducing thorium diideQ-contained in 'said cathode in y'such bide.

8. 'The method of increasing the electron' emissivity 'of a cathode of the thoriated type which 'consists 'in introducing carbon into saideat'hode inan amount less than about one per cent by Weight, heating said cathode in af-non-'eaibonijzingand non-oxidizing nvironme'nt'l to a'temp'erature at whichl a carbide of saidmetal'is formed, activating :said electrodelbyheating'to about 2200O K. and operating'the saine under such conditions 4thatfleoin'bardmentfof-the electrode by positive-ions inappreci'ab-le.

V9. :.The method Lof increasing the electron- :eniissivity of'a cathode o'fthe'thoriated type which consists vin introducing carbon into said electrode lin an amount less than one ,per cent `by vveight,"moun ting` said electrode 'in'a bulb, eva'c'uatng the "same, volatilizin'g inagnesiu'm A:in said -bulb after evacuation, and activating said cathode 'by heating to ian elevated temperature.

`10. lThe'metliod :df vpreparing an lelectrode adaptedfto emit '-e'leetrons at incandescence which consistsiiiflreating a conductor comprising" retractoryv metal and fthoria my acarbonaceous *vapor to 'a -tei'nperature` adapted to cause combination df carbon with said 'conductor` removing fsaid -vapor before the sten 'cathode of. tliethorliated'type 'adaptedfamount'of carbon 'introduced :into thefla# to/einit electrons when "heat-em containing e. a"cathodeempieces-smsnemers at vmeint exceeds one per^eent,^zandheatingsaid earbonfin an appreciable amount `but -les's` vconductor ina'non-oxidizingienviromen#'t0 a temperature-'at whichfelectron -emissivity is'markotlfygncmsed.

11. The method of preparing an electrode adapted to emit electrons at incandescence which consists in heating to incandescence a conductor comprising tungsten and thorla in a carbonaceous vapor until a marked increase in cold resistance occurs by the introduction of carbon, discontinuing the heating before the tungsten has -been largely converted to a carbide, and heating said conductor in the absence of said vapor ina reducing Venvironment to produce a surface layer o f thorium material having a markedl higher electron emissivity than the origina conductor.

l2. Anv electrical discharge device comprising a sealed envelope, electrodes therein which include a cathode. of the thoriated type and a substance, having a marked chemical afiinity forwater vapor, deposited as a film of extensive surface upon the inside of said envelope,sa id substance having a materially lower vapor pressure than the alkali metals. v

13. An electrical discharge device comprising asealed envelope, electrodes thereinl which is ofthe thoriated type and al.

one of film of magnesium deposited on the inner surface of said envelope.

14. A n electron discharge device containing 'a cathode of the thoriated type, said cathode being provided with a reducing agent sufficient'in quantity to promote activation of said cathode, but restricted to an amount which will permit operation of said cathode at a temperature of effective electron. emission with a commercially long life. 15. A cathode of the thoriated type for electric discharge devices containing less {han about three per cent by weight of car- 16. An electric discharge device comprising Yan envelope, electrodes therein one of which comprises a refractory metal, a material reducible by. carbon to` a state of higher electron emissivity than said refractory metal at a given temperature, and a quantity of carbon in an amount insufficient to convert said metal completely to a carbide.

17. An electrical discharge device comprisinga container, electrodes therein one of which contains less than three per cent of carbon combined therewith anda material capable of materially higher electron 'emissivity-'at a givenftemperature than the main constituent of said electrode,but being sensi-` tive to positive ion bombardment, and a quantity of alkaline earth metal deposited in said container out of immediate contact with saidelectrodes.v

18. An electrical discharge ydevice comprising la container, electrodes therein one of which containsza material lcapable of materially higher electron l.emissivity at a given temperature-v thanthe. main .constituent of said electrode, but being sensitive to positive ionrbombardment, and a quantity of alkaline earth meta-l deposited as a film on the inner surface of -said container. i

19. AnV electrical discharge device comprising a container, electrodes therein one of -Which contains a material capable of materially higher electron emissivityat a given temperature 'than the main constituent of said electrode, but being sensitive to positive ion' bombardment,and a quantity 'of magnesium deposited as a film on the inner surface of said container out of immediate contact with said electrodes. I

20. An electrical discharge device comprising a container, electrodes therein one of which consists largely of refractory metal and contains an oxide of arnetal of high electron emissivity which when rendered active is sensitive to ion bombardment together withV a reducing agent, and a quantity of alkaline earth metal, the latter being deposited as a film on the inner wall of said container.

21. An electrical discharge device comprising a glass bulb, electrodes therein one of which consists largely of refractory metal and contains a material capable when activated of substantially increasing the electron emission of the electrode with which it is Avassociated but which vis deteriorated by oxidation and positive bombardment, and a film of magnesium coating the inner surface of said bulb.

22. An electrical discharge device comprising a container, electrodes therein, one of which contains as a constituent a material capable of higher electron emission than the main constituent of said electrode but which is sensitive to positive ion-bombardment, and to the chemical'eiect yof water vapor under operating conditions, and a film of metal deposited from the vapor state in a vacuum on the wall of said containerV and non-carbonizing environment to a temperature suiiiciently high to activate said electrode, and thereupon operating said electrode under conditins which render the ef.

feet of positive ion bombardment of said electrode inappreciable.

24. An electrode material for an electrical discharge device consisting of a metal capable of stable operation at an elevated temsion than `said metal, larid carbon ima quan- 'tity insuiiicient to deleteriously ae'ct the strength of said material, but being capable of re ucng a sucient amount of said compound` to ena-ble said electrode to be more readily activated during use than would .be the casein the absenceof said carbon.

25. A cathode material for'elotrieal dis. 1 charge devices consisting largel of a metal capable of operation at an-eleva d temperature, a reducible compound of a materlal of high electron emissivity and less than about one per cent of carbon. I5 In] witn'lss wllereof, i:IAilave hereunto set m and t is St da o vuflfust 1921.

y IIHNG LAGMUIR.. 

